1. Field of the Invention
This invention relates to composite ultrafine particles of nitrides having a whiskery or columnar structure, a method for the production thereof, and a sintered article obtained by sintering a powder containing the ultrafine particles.
2. Description of the Prior Art
The sintered articles of metal nitrides such as, for example, aluminum nitride are widely used in various applications as to heat-releasing substrates for semi-conductors, printed circuit boards, packaging materials for LSI (large scale integrated circuit), light-pervious heat-resistant materials, etc. because they are possessed of resistance to heat, high thermal conductivity, high electrical insulating quality, and light transmission property. Though these sintered articles of metal nitrides are generally produced by sintering a nitride powder, the properties of the sintered articles thus obtained are appreciably affected by the purity and the particle diameter of a particular powder used as the raw material. The production of a sintered article of high performance, therefore, necessitates the use of a very fine nitride powder of high purity. When such fine ceramic particles are used as a dispersive reinforcing material in a particle-dispersed composite material, they are desired to be very slender particles having a whiskery or columnar structure.
In the production of such ultrafine particles of a metal nitride as mentioned above by the arc plasma method, for example, it is difficult to obtain the ultrafine nitride particles unless the nitride uses such a metal as titanium or zirconium which has strong affinity for nitrogen. The ultrafine particles of TiN or ZrN, however, are at a disadvantage in being producible only in an extremely low yield (about 10 mg/min.).
When aluminum or chromium reputed to produce ultrafine nitride particles in a relatively high yield is used as a vaporizing material, all the produced particles are not nitrided; they are nitrided only up to about 30% in the case of aluminum or about 40% in the case of chromium. Thus, ultrafine particles containing metal and ceramic phases (Al+30% AlN or Cr+40% .beta.--Cr.sub.2 N) are produced.
In view of this destitution, a method for producing a very fine aluminum nitride powder by melting metallic aluminum with an arc or plasma in the atmosphere of a mixed gas consisting of nitrogen and ammonia thereby preparing a very fine mixed powder of metallic aluminum and aluminum nitride and subsequently heating the mixed powder at a high temperature in an atmosphere of nitrogen thereby nitriding the very fine metallic aluminum powder has been proposed (see Japanese Patent Applications, KOKAI (Early Publication) No. 62-283,805 and No. 62-282,635).
This method, however, also has the problem of being incapable of obtaining a sintered article of aluminum nitride of high performance because the produced powder is not wholly nitrided and further because the portion of the very fine metallic aluminum powder which has escaped being nitrided is extremely active and is ignited or oxidized with even a small amount of oxygen. Besides, the produced aluminum nitride particles have relatively isotropic forms such as hexagons or spheres. When this powder is manufactured into a sintered article or it is used as a reinforcing material in the production of a composite material having a metallic matrix, therefore, the sintered article or the composite material has room for further improvement in strength, particularly hot strength.
The production of ultrafine particles by mixing two kinds (metallic aluminum and aluminum nitride) of ultrafine particles having a particle diameter of a micron order in a gaseous phase has been known to the art. The production of composite ultrafine particles having a particle diameter of a nm order by compounding a metal with aluminum nitride has been virtually unknown to the art.